I'll be honest. I never heard of the terms "broad" and "narrow" intertextuality. I googled it and got the definitions you have in reference to Kristeva and Barthes. Clearly, the entire novel is overtly intertextual since it is based on fictional characters in Jane Eyre. As far as narrow intertextuality, (quotations, allusions, etc.), there are plenty to pick from. The burning Coulibri is an allusion (or a kind of post-foreshadowing) to the burning of Thornfield. So, direct metaphors and specific quotes would seem to fall under narrow intertextuality (I think specific or overt is more appropriate).
Broad: (codes, conventions). This is a bit vaguer of course, because it is broad. It's tough because the whole thing is so constantly and consciously intertextual (if you've read Jane Eyre). Even if you haven't, you could say that one broad intertextually strategy is the act of presenting a story from the dual perspective of Caribbean and English. So, in the broadest sense, this is an intertextual link to colonial and postcolonial texts, both from English and Caribbean writers. What I mean is that this dual perspective implores the reader to consider British colonialism and post-colonialism via literary works from both perspectives.
By setting the story in the mid-19th (Jane was set in late 1700s), Rhys introduces intertextual links to more themes (racism, slavery, abolition); issues which are not so present in Jane Eyre. And therefore, those link to other literary texts on those subjects = but those links can only be traced by the reader.
As far as codes, I think this means that since, Rhys' novel brings different themes and perspectives to this conscious intertextual comparison to Jane Eyre, the reader will use those perspectives to, for one example, decode or "unmask" the way Bertha is portrayed in Jane or the way non-whites are portrayed in Victorian British Literature. In other words, use the different perspectives (i.e. Antoinette’s and maybe even Christophine's) to decode the more conventional interpretations of non-whites and colonized peoples. In fact, you don’t even half to look beyond Wide Sargasso Sea itself; you have Rochester there. Are the reader’s interpretations of Rochester’s perception of non-whites the same in both novels? Or, how is the reader’s interpretation of Bronte’s conventions of a governess compare to Christophine’s role, which seems to be similar to that of a governess.